A latent electrostatic image can be developed with toner particles dispersed in an insulating non-polar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are also known for forming latent electrostatic images such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. After the latent image has been formed, the image is developed by the colored toner particles dispersed in the non-polar liquid. The image may then be transferred to a receiver sheet.
Useful liquid developers comprise a thermoplastic resin and a dispersant non-polar liquid. Generally, a suitable colorant, such as a dye or pigment, is also present. The toner particles are dispersed in the non-polar liquid which generally has a high volume resistivity in excess of 10.sup.9 ohm-centimeters, a low dielectric constant (i.e. below 3.0) and a high vapor pressure. Generally, the toner particles have a particle size range of 0.01 to less than 10 .mu.m, and have an average by area particle size of less than 10 .mu.m as measured by the Horiba CAPA-500 centrifugal automatic particle analyzer, manufactured by Horiba Instruments, Inc., Irvine, Calif. (solvent viscosity of 1.24 cps; solvent density of 0.76 g/cc; sample density of 1.32 using a centrifugal rotation of 1,000 rpm), and an average by area particle size of less than 30.mu.m as measured by a Malvern 3600E Particle Sizer, manufactured by Malvern, Southborough, Mass.
The Malvern 3600E Particle Sizer uses laser diffraction light scattering of stirred samples to determine average particle sizes. Because the Horiba and Malvern instruments use different techniques to measure average particle size the readings differ. The following correlation of the average size of toner particles in micrometers (.mu.m) for the two instruments is:
______________________________________ Value Determined By Expected Range For Malvern 3600E Particle Sizer Horiba CAPA-500 ______________________________________ 30 9.9 .+-. 3.4 20 6.4 .+-. 1.9 15 4.6 .+-. 1.3 10 2.8 .+-. 0.8 5 1.0 .+-. 0.5 5 0.2 .+-. 0.6 ______________________________________
The correlation is obtained by statistical analysis of average particle sizes for 67 liquid electrostatic developer samples (not of this invention) obtained on both instruments. The expected range of Horiba values was determined using a linear regression at a confidence level of 95%.
Because the formation of proper images depends on the difference of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and preferably other adjuvants which increase the magnitude of the charge (e.g., polyhydroxy compounds, aminoalcohols, polybutylene succinimide compounds, aromatic hydrocarbons, metallic soaps, etc.) to the liquid developer comprising the thermoplastic resin, the non-polar liquid and the colorant. The properties of the toner particles are critical to the optimal electrical functioning of the liquid electrostatic developer and the production of good quality images.
U.S. Patent No. 5,019,477 to Felder discloses a liquid electrostatic developer comprising a non-polar liquid, thermoplastic resin particles, and a charge director. The thermoplastic resin particles comprise a mixture of (1) a polyethylene homopolymer or a copolymer of (i) polyethylene and (ii) acrylic acid, methacrylic acid or alkyl esters thereof, wherein (ii) comprises 0.1-20 weight percent of the copolymer; and (2) a random copolymer of (iii) selected from the group consisting of vinyltoluene and styrene and (iv) selected from the group consisting of butadiene and acrylate. As the copolymer of polyethylene and methacrylic acid or methacrylic acid alkyl esters, Nucrel.RTM. may be used.
U.S. Pat. Nos. 3,852,208 and 3,933,664, both to Nagashima et al., disclose colored, light-transparent photo-conductive material which is obtained by a condensation reaction of organic photoconductive substances with reactive colored components. The chemical combination of an organic photoconductive substance having at least one amino or hydroxyl group with a color development component having at least one active halogen atom produces the color developing organic photoconductive materials. Alternatively, the color developing materials can be obtained from the chemical combination of an organic photoconductive substance having at least one active halogen atom with a color developing component having at least one amino or hydroxyl group. The color developing organic photoconductive material may be pulverized in a ball-mill, a roll-mill or an atomizer to produce a toner for use as a dry or wet developing agent, or may be used in combination with other colored substances or vehicle resins.
U.S. Pat. No. 4,524,119 to Luly et al. discloses electrophotographic development carrier particles for use with toner particles wherein the carrier core particles are coated with fluorinated carbon or a fluorinated carbon-containing resin. By varying the fluorine content of the fluorinated carbon, systematic uniform variation of the resistivity properties of the carrier is permitted. Suitable binders for use with the carrier core particles may be selected from known thermoplastics, including fluoropolymers.
U.S. Pat. No. 5,026,621 to Tsubuko et al. discloses a toner for electrophotography which comprises as main components a coloring component and a binder resin which is a block copolymer comprising a functional segment (A) consisting of at least one of a fluoroalkylacryl ester block unit or a fluoroalkyl methacryl ester block unit, and a compatible segment (B) consisting of a fluorine-free vinyl or olefin monomer block unit. The functional segment of block copolymer is oriented to the surface of the block polymer and the compatible segment thereof is oriented to be compatible with other resins and a coloring agent contained in the toner, so that the toner is provided with both liquid-repelling and solvent-soluble properties.
U.S. Pat. No. 5,030,535 to Drappel et al. discloses a liquid developer composition comprising a liquid vehicle, a charge control additive and toner particles. The toner particles may contain pigment particles and a resin selected from the group consisting of polyolefins, halogenated polyolefins and mixtures thereof. The liquid developers are prepared by first dissolving the polymer resin in a liquid vehicle by heating at temperatures of from about 80.degree. C.-120.degree. C., adding pigment to the hot polymer solution and attriting the mixture, and then cooling the mixture so that the polymer becomes insoluble in the liquid vehicle, thus forming an insoluble resin layer around the pigment particles.
U.S. Pat. No. 4,797,341 to Tsubuko discloses a liquid two toner component developer system. The first component is small particles of toner resin containing a dye or pigment. The resins disclosed for the first component are copolymers of alkyl acrylates or methacrylates and divinylbenzene or its alkyl derivatives. Optionally, other monomers may also be present. The second component consists of larger particles of a pure polytetrafluoroethylene (PTFE) resin and a colorant. The larger particles act as carriers for the smaller particles.
U.S. Pat. No. 3,761,413 to Hulse discloses a dry xerographic toner composition consisting of a mixture of resinous particles having a colorant uniformly dispersed in the particle and a small amount of finely divided polytetrafluoroethylene or of finely divided copolymer of tetrafluoroethylene and hexafluoropropylene. The toner composition is prepared by polymerization. Molecular weight range and melt index range are controlled by adjusting the amount of polymerization initiator and the temperature employed in the polymerization.